Fish Disease/Diagnosis and Proper Treatment

Whether you are a beginner or a seasoned marine reef hobbyist you will most likely encounter fish disease in your reef system. The trick, is to find the problem as soon as possible, recognize the disease and to properly treat it with the right medication. Sometimes this is a difficult task. When most hobbyists see a problem they treat it with the wrong medication, and the result is usually fish loss and well earned money down the drain; not to mention the suffering of the fish.

In this segment, I would like to focus on the most common causes of fish loss which are parasites and bacteria. Usually parasites come first and bacteria second. The reason for this, is parasites feed on the fish and end up leaving lichens (open wounds), which in time get infected and stress the fish. This stress causes more parasites and it becomes a never ending cycle. That is why it is important to treat the bacterial infection as well as the parasites.

You may ask, why is it when the fish are at the store or in my hospital tank there are no parasites, but when I introduce them in my system the parasites magically appear? The reason for this could be as simple as the stress from transfering the specimen from the store to your home, or from the hospital tank to your display tank. If this is the case it will be easy to treat with a reef safe medication or in some instances they will cure themselves. Remember, as long as the fish are in copper, parasites will not be visible and an untrained eye may purchase the specimen not knowing the problem that may lay ahead. That is why it is very important before you purchase or treat one of your pre-existing fish to look for abrasions on the body and also discoloration of the fish or cloudy/hazy eyes. If these signs are present the fish most likely has, or will soon encounter a bacterial infection that will cause a major outbreak of Ich. This will not only jeopardize the fish infected, but all the fish in your show tank. Remember, parasite medication will not treat a bacterial infection nor will bacteria medication (antibiotic) rid your fish of parasites.

We at Chem-Marin have been testing erythromycin in reef systems for the last 4 years all over the U.S. with a 99% success rate and with out harming the most delicate species, including corals invertebrates, fish and your bacteria bed as long as your system is a year or more old. If your system is new you can still treat with erythromycin, but careful monitoring of ammonia is critical as your bacteria bed is not at a mature state. This means that even though the gram negative of your bioload bacteria has cycled, it has quality but does not have quantity. Quantity is important as erythromycin will also knock out some of your beneficial bacteria (gram negative) which helps to sustain life in your reef system.

Erythromycin takes care of gram positive and gram negative bacteria safely and effectively. Below is a method that we found most effective and safe that we would like to share with you to help you have the success that you and your fish deserve.


For every 30 gallons of water use 300mg of erythromycin. Use once a day for 4 days in a row and then every other day for a total of 9 treatments. If symptoms are still visible skip 2 days and readminister for 9 more treatments. Watch for skimmer overflow at all times of treatments, and check water perimeters while administering.